Water Quality and Animal Feeding Operations in Arizona: A Producer's Notebook

25-year, 24-hour Precipitation Events for Arizona

A 25-year, 24-hour storm event means the maximum 24-hour precipitation event with a probable recurrence interval of once in 25 years, as defined by the National Weather Service.  

For Arizona, the range is approximately 2.5 inches to 6.0 inches in a 24-hour period.  In order to comply with state regulations (see page 3-2, A.A.C. R18-9-403) and the Arizona NPDES General Permit for CAFOs, containment facilities must have capacity to hold all process wastewater, manure water and contaminated stormwater up to a 25-year, 24-hour precipitation event.

A map showing the 25-year, 24-hour precipitation event is included in this notebook (small version shown) and may also be viewed, full size at http://www.wrcc.dri.edu/pcpnfreq/az25y24.gif.

Map of a 25-year, 24-hour precipitation event

Page updated 7/02

Acronyms and selected definitions

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